This invention relates to electronics, and more particularly to compositions useful for producing conductor patterns adherent to substrates.
Conductor compositions which are applied to and fired on dielectric substrates (glass, glass-ceramic, and ceramic) usually comprise finely divided inorganic powders (e.g., metal particles and binder particles) and are commonly applied to substrates using so-called "thick film" techniques, as a dispersion of these inorganic powders in an inert liquid medium or vehicle. Upon firing or sintering of the film, the metallic component of the composition provides the functional (conductive) utility, while the inorganic binder (e.g., glass, crystalline oxides such as Bi.sub.2 O.sub.3, etc.) bonds the metal particles to one another and to the substrate. Thick film techniques are contrasted with thin film techniques which involve deposition of particles by evaporation or sputtering. Thick film techniques are discussed in Handbook of Materials and Processes for Electronics, C. A. Harper, Editor, McGraw-Hill, N.Y., 1970, Chapter 12.
The most commonly used conductor compositions employ noble metals, especially gold, silver, platinum, palladium, and their mixtures, alloys, and compounds, since their relatively inert characteristics permit firing in air. Attempts at using dispersions of less expensive non-noble metals have often been confined to specialized uses or have required the great practical inconvenience and expense of firing in non-oxidating atmospheres (nitrogen, nitrogen/hydrogen, hydrogen, argon, etc.).
The prior art on conductor compositions includes the use of glass binders for noble metals, as well as the use of glass-free binders, such as the copper oxide binder (with gold) mentioned by Gucker and Treptow (Am. Ceram. Sc. Bull. 46, 789, 1967), and by Smith in U.S. Pat. No. 3,799,890. Glass-free copper oxide/cadmium oxide binder was used with gold in Smith U.S. Pat. No. 3,799,891. Sheard U.S. Pat. No. 3,763,409 discloses glass-free compositions of palladium or palladium oxide plus copper oxide. Compositions of silver/copper oxide and silver/copper oxide/glass have been available commercially for about 15 years.
Martin U.S. Pat. No. 3,293,501 discloses conductive films of glasses comprising copper and bismuth oxides. Buck U.S. Pat. No. 3,776,769 discloses compositions of noble metal, copper oxide and glass, fireable in reducing atmospheres. White U.K. Pat. No. 855,625 discloses conductive films of silver, copper oxide, and an "inert" material such as alumina, magnesia, zirconia, titania, chromic oxide or aluminasilicate. Short U.S. Pat. No. 2,819,170 discloses compositions of silver and a vitrifiable flux of bismuth oxide and cadmium borate.
The more expensive noble metals (Au, Pt, Pd) provide more stable fired conductors. Despite its instability relative to the other noble metals, silver has been used either alone or with Pt or Pd to produce less expensive conductors. To increase adhesion to the substrate of such silver conductor compositions, the relative amount of inorganic binder is increased. However, often the more binder present, the lower the conductivity. There is a need for inexpensive silver-based compositions which can be fired as low as 850.degree.C. in air to produce conductors exhibiting good aged adhesion to the substrate after soldering, both after being aged at room temperature or 150.degree.C. for a day or two, and also have good soldering characteristics. Of special interest are conductor compositions of at least 95% metal (5% or less binder), which despite low binder levels exhibit relatively less sensitivity to variations in firing temperature, greater processing latitude. Also, firing capability at 850.degree.C. is especially desirable; prior art compositions are often fired as high as 950.degree.C.